docs: def pending_confirmations(self):
          """Return all published messages that have yet to be acked, nacked, or
          returned.
  
          :return: [(int, Published)]
  
          """
          return sorted([(idx, msg)
                         for idx, msg in enumerate(self.published_messages)
                         if not msg.future.done()],
                        key=lambda x: x[1].delivery_tag)